J. AMER. SOC. HORT. SCI. 120(6):977-982. 1995. Response of Forsythia × itermedia ‘Spectabilis’ to Uniconazol. I. Growth; Dry-matter Distribution; and Mineral Nutrient Content, Concentration, and Partitioning Mack Thetford1, Stuart L. Warren2, and Frank A. Blazich3 Department of Horticultural Science, North Carolina State University, Raleigh, NC 27695-7609 Additional index words. sumagic, plant growth retardant, triazole, gibberellin biosynthesis inhibitor Abstract. Uniconazole was applied as a foliar spray at 0, 90, 130, 170, or 210 mg·liter –1 to rooted stem cuttings of ‘Spectabilis’ forsythia (Forsythia ×intermedia Zab.) potted in calcined clay. Plants were harvested 0, 40, 80, 120, and 369 days after treatment (DAT). Treatment with uniconazole at 90 to 210 mg·liter–1 suppressed leaf area and dry weight an average of 16% and 18%, respectively, compared to the nontreated controls when averaged over all harvest periods. Stem and root dry weight suppression was greatest at 80 DAT, 47% and 37%, respectively. Uniconazole suppressed root length from 15% to 36% and root area from 15% to 33% depending on harvest date. Internode length and stem diameter of uniconazole-treated plants were suppressed at all harvests except 369 DAT. Uniconazole resulted in increased and decreased root : shoot ratios 40 and 80 DAT, respectively; while root : shoot ratios were not affected for the remainder of the study. Relative growth rates of leaves, stems, and roots decreased with increasing uniconazole concentration; however, no relative growth rates were suppressed beyond 80 DAT. Generally, mineral nutrient concentrations increased as a result of uniconazole application. The proportion of mineral nutrients allocated to leaves and roots was not affected while the proportion of nutrients allocated to stems decreased with uniconazole application compared to the controls. Chemical name used: (E)-1-(p-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol (uniconazole). Increasing costs and restrictions on disposal of organic wastes Foliar application of paclobutrazol resulted in increased root tip in landfills has stimulated interest in the use of plant growth diameter and decreased nonsuberized root length of ‘Redhaven’ retardants to limit vegetative growth and reduce pruning frequency peach [Prunus persica (L.) Batsch] grown in a commercial potting of woody landscape plants. Research has demonstrated the poten- mix (Williamson et al., 1986) while similar responses were not tial use of uniconazole (Valent U.S.A., Walnut Creek, Calif.), as an evident with ‘Nemaguard’ peach rootstock [P. persica × Prunus economical method of reducing plant growth in established land- davidiana (Carrie're) Franch.] following continuous paclobutrazol scape plantings (Keever and West, 1992) or as a component of exposure in nutrient solution culture (Rieger and Scalabrelli, container production regimes for woody landscape species (Norcini 1990). In addition, foliar application of paclobutrazol increased and Knox, 1989; Warren 1990; Warren et al., 1991). weight, diameter, and length of fibrous roots of seedlings of apple Uniconazole is a member of a group of compounds known as (Malus domestica Borkh.) (Steffens et al., 1983). Such changes triazoles which retard growth of several ornamental species (Bailey may also affect water and mineral nutrient uptake since these and Miller, 1989; Keever and West, 1992; Norcini and Knox, processes occur principally through nonsuberized portions of 1989; Wang and Gregg, 1989; Warren 1990). Triazoles are xylem young roots. mobile and when applied to roots, young stems, and to a lesser Research with paclobutrazol involving root zone or bark applica- extent the youngest leaves, are absorbed and translocated acro- tion to tree fruits suggests this compound can alter mineral nutrient petally to the leaves (Reed et al., 1989; Sterrett, 1988). Uniconazole status. Mineral nutrient content and concentration responses, how- limits plant growth by inhibiting gibberellin biosynthesis (Izumi et ever, differed with the species tested and method of application al., 1984). Several effects other than suppression of shoot growth (Atkinson, 1986; Atkinson and Crisp, 1982; Rieger and Scalabrelli, have also been reported and include suppressed leaf area, altered 1990; Swietlik and Miller, 1985; Wang et al., 1985; Wieland and root morphology and physiology, and altered root : shoot ratios Wample, 1985). Currently, little information is available concerning (Davis et al., 1988). the effects of triazoles on root growth of woody landscape species Conflicting results have been reported concerning changes in (Reed et al., 1989) or the effects root system alteration may have on root growth following application of paclobutrazol (another triazole mineral nutrient status of such plants. compound) as a foliar spray or in a nutrient solution to tree fruits. While these above mentioned reports provide useful informa- tion about the effectiveness of uniconazole following a soil or potting medium application or inclusion within a nutrient solution Received for publication 16 Dec. 1994. Accepted for publication 8 May 1995. This culture, these application methods allow continuous exposure to research was funded by the North Carolina Agricultural Research Service (NCARS), uniconazole through root uptake. Triazoles can be quite persistent Raleigh, NC 27695-7643. Use of trade names in this publication does not imply endorsement by the NCARS of products named nor criticism of similar ones not with growth retardant effects occurring on plants that were grown mentioned. Uniconazole was provided by Valent U.S.A., Walnut Creek, Calif. on soil treated the previous year (Fletcher and Hofstra, 1985); an Technical assistance of William M. Reece and the Analytical Service Laboratory, undesirable characteristic for plants destined for the landscape. Dept. of Soil Science, is gratefully acknowledged. This paper is based on a PhD There also exists major differences in effectiveness among the thesis by M.T. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby gibberellin biosynthesis inhibitors which result in differences in marked advertisement solely to indicate this fact. longevity of effect and extent of inhibition; uniconazole is more 1Former graduate teaching assistant. Currently assistant professor, Univ. of Florida, effective than paclobutrazol, flurprimidol, and triadimefon in 5988 Hwy. 90 W., Bldg. 4900, Milton, FL 32583. reducing growth (Steffens, 1988). Before uniconazole can be 2Associate professor. 3 incorporated into a production or maintenance regime, a better Professor. J. AMER. SOC. HORT. SCI. 120(6):977-982. 1995. 977 understanding of how the product may affect other production or Leaf area (entire plant) was determined at each harvest with a leaf maintenance practices is needed. Previous research has demon- area meter (LI-3000; LI-COR, Lincoln, Neb.). Stems and roots strated vigorous shoot growth of ‘Spectabilis’ forsythia can be were separated after washing the substrate from the root systems. suppressed with foliar (leaf and stem) application of uniconazole, A subsampling technique was developed to estimate total root but this work did not consider the effect uniconazole may have on area and total root length of the extensive, highly branched root root growth (Warren et al., 1991). Therefore, the objectives of this systems. At 40 DAT, root length and area of the entire root system research were to determine if 1) uniconazole-induced shoot sup- and a root subsample were determined from 11 additional nontreated pression is accompanied by root growth suppression with subse- plants from within the guard rows. Each root subsample consisted quent changes or alterations in dry matter distribution between of two roots originating from the root collar (stem-root junction). roots and shoots, and 2) uniconazole application influences min- Prior to measurement, all roots were weighed and then soaked in eral nutrient content, concentration, and partitioning within 1% methylene blue for 48 h. Root area and root length were ‘Spectabilis’ forsythia. measured with an image analyzer (Digital Image Analysis System; Decagon Devices, Inc., Pullman, Wash.). Root subsample and root Materials and Methods fresh weights were then used to estimate total root area and total root length. Experiment 1 (first outdoor container study). Rooted stem cuttings of ‘Spectabilis’ forsythia were potted on 14 May 1991 into 3.8 liter containers with arcillite, a calcined clay substrate (Aimcor Inc., Deerfield, Ill.) and placed on a gravel container pad at the Horticulture Field Laboratory, Raleigh. Arcillite was selected as a substrate to allow complete recovery of intact root systems at harvest. Uniconazole (500 mg·liter–1 solution formulation) at 0, 90, To determine the ability-of the root subsampling technique for 130,170, or 210 mg·liter–1 was applied to forsythia on 13 June 1991 estimating actual total root length and total root area, estimated between 7:00 AM and 8:30 AM with a hand held sprayer to uniformly root area was regressed on actual root area and estimated root wet all leaves and stems (12 to 15 ml per plant). These concentra- length was regressed on actual root length resulting in the follow- tions were chosen on the basis of results of previous experiments ing linear equations: which evaluated the effectiveness of a 500 mg·liter–1 solution and Actual total root area (cm2) = a 30% WP